1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method. More particularly, the invention relates to a high image quality photothermographic material with high sensitivity and preferable gradation and an image forming method using the same.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for providing a dry photographic process from the viewpoints of protecting the environment and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by transmitting it to a desired location. Here the image information is output onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using pigments or dyes, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording materials, images on the digital imaging recording materials obtained by such general image forming systems are insufficient in terms of the image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses, and high recording speeds (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
Photothermographic materials utilizing organic silver salts are already known. Photothermographic materials have an image forming layer in which a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder.
Photothermographic materials form a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. Photothermographic materials are described in “Thermally Processed Silver Systems”, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. M. Sturge, V. Warlworth, and A. Shepp, chapter 8, pages 279 to 291, 1989, and the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system that has been made commercially available.
Since this kind of image forming system utilizing an organic silver salt has no fixing step, undeveloped silver halide remains inside the film after thermal development. Thus, there have intrinsically been two serious problems in the system.
One of them involves image instorability after a thermal developing process, particularly fogging due to print-out when the material is exposed to light. As a means to improve print-out, a method of using silver iodide is known. Silver iodide has the characteristic of causing less print-out than silver bromide or silver iodobromide having an iodide content of 5 mol % or less, and has a potential for fundamentally solving the problem. However, the sensitivity of silver iodide grains known until now is extremely low, and the silver iodide grains do not achieve a level of sensitivity that is applicable for an actual system. When means of preventing recombination between photoelectrons and holes is performed to improve the sensitivity, it is an inherent problem that the characteristic of being excellent in the print-out property will be lost.
As means of increasing the sensitivity of a silver iodide photographic emulsion, the literature discloses addition of a halogen acceptor such as sodium nitrite, pyrogallol, hydroquinone or the like, immersion in an aqueous silver nitrate solution, sulfur sensitization at a pAg of 7.5, and the like. However, the sensitization effect of these halogen acceptors is very small and extremely insufficient for use in photothermographic materials.
Another problem is that light scattering due to the remaining silver halide grains may cause cloudiness whereby the film turns translucent or opaque and image quality is degraded. To solve this problem, means in which the grain size of photosensitive silver halide grains is made fine (to within a range of practical use of 0.08 μm to 0.15 μm) and the addition amount is reduced as much as possible to suppress the cloudiness caused by the silver halide have been practically employed. However, the compromise results in decreasing the sensitivity further, the problem of cloudiness is not completely solved, and a dark milky color continues to remain and generate haze in the film.
In the case of a conventional wet developing process, the remaining silver halide is removed by processing with a fixing solution containing a silver halide solvent after the developing process. For the silver halide solvent, many kinds of inorganic and organic compounds are known which can form complexes with silver ions.
Even in the case of a dry thermal developing process, many attempts to introduce similar fixing measures in the material have been made. For example, a method has been proposed where a compound capable of forming complexes with silver ions is incorporated in the film and the silver halide is solubilized (usually referred to as fixing) through thermal development. However, this proposal only applies to silver bromide and silver chlorobromide, and the process also requires an additional heat treatment step for fixing, and the heating conditions require a high temperature within a range of from 155° C. to 160° C. Thus, the system is one in which fixing is difficult to achieve. In another proposal, a separate sheet (referred to as a fixing sheet) that includes a compound able to form complexes with silver ions is prepared, and after thermally developing the photothermographic material to form an image, the fixing sheet is overlaid on the developed photothermographic material, heating is carried out, and the remaining silver halide is dissolved and removed. However, since this proposal requires two sheets, from a practical viewpoint the obstacles are that the processing step is complicated and the operational stability of the process is hard to maintain, and that there is a necessity to discard the fixing sheets after processing, resulting in generation of waste.
As another fixing method usable in thermal development, a method is proposed where a fixing agent for the silver halide is encapsulated in microcapsules, and thermal development releases the fixing agent and causes it to act. However, it is difficult to achieve a design that effectively releases the fixing agent. A method for fixing using a fixing solution after thermal development is also proposed, but it requires a wet process and therefore is not adequate for a completely dry process.
As described above, known methods for improving the turbidity of film have negative effects, and there have been substantial difficulties in their practical application.
On the other hand, attempts have also been made at applying the above-mentioned photothermographic material as photosensitive material for photographing. The term “photosensitive material for photographing” as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a lens, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films, mammography films and the like, various kinds of photomechanical films used in printing, industrial recording films, films for photographing with general-purpose cameras, and the like. For example, an X-ray photothermographic material coated on both sides using a blue fluorescent intensifying screen, a photothermographic material containing tabular silver iodobromide grains described in Japanese Patent Application Laid-Open (JP-A) No. 59-142539, and a photosensitive material for medical use containing tabular grains that have a high content of silver chloride and have (100) major faces, and that are coated on both sides of a support, which is described in JP-A No. 10-282606, are known. However, according to these disclosed examples, fine particle silver halide grains having a grain size of 0.1 μm or less are used, and the sensitivity is very low. Therefore, it has been difficult to obtain a sensitivity required for use in photographing. On the other hand, in the case of using silver halide grains having a grain size of 0.5 μm or more, because the remaining silver halide may increase the haze and worsen the print-out, deterioration of the image quality is severe, and the grains are not applicable for practical use.
The problem of low sensitivity has been a fundamental problem and a significant obstacle, and therefore it was largely unclear what other problems existed for practical use.